Direct Liquid Injection

It is possible to directly meter liquid flow into a vaporizer and then deliver the resulting vapor with or without a carrier gas. This approach also minimizes the time the liquid precursor spends at high temperature, useful for unstable chemistries. There are several approaches...

Displacement Pumps

Displacement pumps (piston in cylinder) can be used, but in order to avoid interruption of the flow while refilling the cylinder, two cylinders are used. One is twice the size of the other and refills cylinder 2 while it delivers liquid. The liquid flows to a vaporizer chamber, e.g. a stack of thin metal disks or a bundle of metal wires, past which flows carrier gas. This arrangement ensures heating of the liquid and evaporation into the carrier. Note that the use of direct liquid injection means the amount of vapor delivered is not sensitive to the exact temperature of the vaporizer.

In practice, it is very difficult to get the two cylinders exactly matched. The liquid delivery pressure to the vaporizer can be monitored to check for consistency.

Liquid Mass Flow Controller

A thermal mass flow controller can be used for a liquid just as for a gas. However, a problem arises: the flow rates for the liquid are typically extremely low, due to the tremendous volume expansion occurring in vaporization (200:1 to 1000:1 is typical). To deliver 10 sccm of vapor one might require a liquid flow of 0.05 sccm.

The liquid is typically pressurized to flow to the vaporizer using a carrier gas. If the carrier gas dissolves into the liquid, bubbles may nucleate at the vaporizer (where the pressure falls), causing sporadic flow disruptions. This can be prevented using a degas stage, in which the liquid is passed through e.g. a thin Teflon tube. The Teflon is porous enough to allow dissolved gas to escape but prevents liquid from leaking out.

Vaporizer

A vaporizer is typically a fritted material or a set of flat metal plates through which liquid flows. Since the liquid is evaporating, any involatile contaminants in the liquid accumulate in the vaporizer and may eventually clog the pores. Vaporizers must be cleaned periodically. A fine capillary tube can be used as a vaporizer ( "Apparatus for Fluid Delivery in Chemical Vapor Deposition Systems" C. Fan, A. Pearson, J. Chen, J. White US Patent 5,620,524 April 15, 1997).

Excessive liquid flows can overload the vaporizer, causing unvaporized liquid to escape into the process piping with consequent instability in delivered vapor.